The present invention pertains to the field of wireless data communications systems, and more specifically to a system and method for non-real time communication between a fixed base station and a plurality of remote transceivers.
Wireless communications have become a principal means for exchanging data with geographically dispersed data collection or interfacing devices, such as utility meters or inventory control monitors. This significantly reduces the amount of time and manpower that were previously required for such data collection activities. However, as the number of remote devices increases, a single data conduit, such as a narrow frequency channel, becomes inadequate to carry the amount of information that must be transferred.
In the area of wireless two-way data communications between a base station (BS) and a plurality of remote receiver/transmitters (RTs), optimum use of the data channel is difficult to obtain due to both signal degradation characteristics of the transmission medium and the message collisions that occur when multiple RTs attempt to transmit on the same channel at the same time. Typical implementations involve a time-division-multiplexed architecture (TDMA) wherein the available time is partitioned between the plurality of RTs that are using the channel, such that each RT will have uncontested use of the channel for a finite period of time. As the number of the RTs using the channel grows, however, such an approach necessitates the reduction in the time allocated to each RT, resulting in a reduction in the data that can be transmitted. To alleviate such bandwidth reduction, various compression techniques are typically employed; however, these techniques add processing time at both ends of a communications link.
Due to the high number of errors that occur during high speed over-the-air data transmissions, sophisticated forward error correction (FEC) processing is needed to accurately reconstruct degraded messages. Typically, such FEC implementations add additional data bytes to the message, which uses up some of the bandwidth gained from the data compression. Further, common FEC algorithms, such as variations of Reed-Solomon, require significant processing time to make such data corrections, which significantly reduce battery life in portable RTs.
When the bandwidth of a communications channel becomes restricted, larger data messages and/or large numbers of transceivers cannot be used in a network without excessive message latency. One solution is to employ various interactive control techniques which allow multiple transceivers to use the same time slot. Typically, such techniques involve including specific RT identification (ID) addresses in a message to direct one or more RTs to process the remainder of the message and to transmit a return message at a specific future time. Such a return message might be an acknowledgement of satisfactory reception of the received message or a request for retransmission of all or a part of the message that could not be reconstructed.
Further, transmission time is partitioned into packets of typically one to 20 seconds in duration which are in turn partitioned into individually addressed messages. Such packet partitioning is usually required due to the need for physical adjustments of the transmission and receptions means, such as periodic data resynchronization of receivers, cooling of transmitters, etc. This partitioning allows all RTs operating on a common channel to examine a smaller portion of each message for a unique ID indicating that the RT is one of the intended recipients of the message. All other RTs monitoring the same message can return to a low power state without processing the remainder of the message, thus reducing undesired RT battery consumption. The drawback of such partitioning is that the ID address must be sent unprotected by FEC encoding leading to errors in the ID addresses.
For RT applications requiring a time mark or other group command, such as the initiation of a data logging process at a plurality of data sensing devices, a first portion of any ID address is typically a group identifier command which directs all RTs of that group to process the message. For other applications which require the uploading of data blocks, individual RTs will have to be addressed with a transmission time and block size allocation. These up-link transmissions can be timed to span several packets, and such applications are controlled by a scheduling means in the BS-computer.
A significant drawback of such a scheduling architecture is that another group of applications, such as application monitors and alarms, cannot gain immediate access to a channel, but cannot wait for a routinely scheduled transmission time slot for the RT. For such applications, time slots are usually reserved in the up-link packet, during which RT requests for transmission time are allowed. To avoid significant system inefficiencies, such time slots are kept small. However, this small time size can be detrimental if a large number of RTs initiate a request in the same time slot, in that two transmitted messages will collide and cancel each other.
Thus over-the-air two-way data communications systems having high data rates and a large number of RTs suffer significant drawbacks between the competing requirements of data accuracy, bandwidth availability and efficiency, and portable RT power consumption.
These and other problems are addressed by the present invention which comprises a system and method for non-real-time two-way wireless data communications. The system has a fixed base station (BS) and a plurality of remote receiver/transmitters (RTs) embodying a BS controller which dynamically selects the operating characteristics of the communications system and provides for efficient use of available bandwidth to maximize the number RTs capable of using a given channel. By dynamically changing a plurality of data blocks in a data frame, a plurality of data rates, a plurality of signal modulation techniques, and plurality of frequencies according to a computer analysis of the received signals, an optimum communications network is realized for a plurality of RT-linked applications.
An object of the present invention is to reduce the quantity of data transmitted in a forward link from a BS to a plurality of RTs through the use of abbreviations of the plurality of RT identification addresses (IDs), use of wildcards in the ID character fields to enable calls to groups of RTs, and concatenation (or head-to-tail joining) of a plurality of data blocks. Concatenation advantageously eliminates the need to fill messages with dummy data bytes to attain the fixed data block size which is typically required by sophisticated forward error correction (FEC) algorithms. Thus, for a serial data stream comprised of a plurality of fixed size error correction blocks, each FEC data block can include a plurality of concatenated messages. Alternatively, a single message can span several FEC data blocks, and only the trailing FEC data block may need to be filled with dummy data bytes.
Another object of the present invention is to provide a method for optimization of each of the plurality of RTs, wherein a plurality of data blocks are sequentially transmitted and each data block is modulated by a different one of a plurality of modulation techniques which are; arranged in ascending order from the simplest to the most sophisticated.
Another object of the present invention is to provide methods for avoiding message collision in a two way wireless communications system, wherein a plurality of RTs attempt to initiate unscheduled transmissions simultaneously. A first method employs a randomly generated number to select one of a plurality of possible time periods within a fixed transmission time period. Each RT from the plurality of RTs generates a unique random number each time it attempts an unscheduled transmission.
A second method uses a weighting equation which includes a message priority, a number representing the transmission attempts, and a random number.
A third method uses an expanded transmission time period. This method is used for those RTs which have exceeded a predetermined maximum number of attempted transmissions of the same message.
Another object of the present invention is to use variable sized data blocks for messages and message acknowledgements to improve system efficiency.
Another object of the present invention is to provide a method for using a fixed group of emergency alarm time periods each having a priority and wherein alarms are generated in one or more of the plurality of RTs.
Another object of the present invention is to provide a method for dynamically monitoring received signal strength to determine the presence of multiple transmission signals occurring within the same time period, which would indicate the corruption of a message.